1. Field of the Invention
The present invention is directed to a process for the production of a base foil of an aluminum alloy that permits formation of an aluminum alloy foil that is highly strong and substantially free of rib-like patterns on both of its surfaces from all appearances.
2. Description of the Related Art
For their softness in character and rolling with ease, aluminum alloys have been applied, after being rolled to a thickness of approximately 5 to 150 xcexcm, as aluminum alloy foils for wrapping of for example, foodstuffs, medicines, tobaccoes and so on.
Such aluminum alloy foils have been used in single-layered form or in multi-layered form in combination with paper, resin film or the like. Meanwhile, foils composed solely of an aluminum element and stipulated as JIS Type 1000 are limited in regard to their applications. For this reason, those foils of an Alxe2x80x94Fe alloy type containing Fe in an amount of about 0.3 to 1.5% by weight have today taken the place of the all-aluminum foils.
The Alxe2x80x94Fe alloy foils are produced by the steps of coating the associated hot melt, through a semi-continuous casting method, into a cast plate in a thickness in the order of 500 mm, heating the cast plate at an elevated temperature to thereby effect uniform heat treatment, hot rolling, cold rolling and intermediate annealing so that a base foil is prepared with a sheet thickness of about 0.3 mm. The base foil is finally rolled into a final foil of about 5 to 150 xcexcm in thickness. In the case of a foil with a thickness of 5 mm, two intermediate foils derived at a stage just before the final stage are rolled in superimposed relation to each other.
However, the semi-continuous casting method involves segregation during casting, thus requiring not only surface planing in the range of about 5 to 10 mm and heat treatment for homogenization at from 500 to 600xc2x0 C. and the like, but also hot rolling to reduce a cast plate of about 500 mm to a thickness of about 6 mm. Foil production using such a semi-continuous casting method has the drawback that it gives rise to decreased yield as well as added process steps and hence tedious production control.
On the other hand, a certain continuous casting method has been proposed in which an ingot is directly cast into a slab of 10 to 30 mm in thickness, which slab is continuously subjected to hot rolling. This continuous casting method has good productivity because it completes solidification at a higher speed than the semi-continuous casting method, thus entailing least segregation while in casting and enabling omission of surface planing of a cast lump of 500 mm in thickness and also of rolling permits the casting to a thickness of about 10 to 30 mm.
However, when an aluminum alloy foil is produced by the above continuous cast-rolling method, foil rolling gets diminished. To obviate this problem, it has been taught as in Japanese Unexamined Patent Publication No. 6-93397 that an aluminum alloy sheet containing Fe and Si and resulting from the continuous casting rolling method can be heat-treated twice so as to form a cold-rolled sheet. Advantageously, this treatment brings about reduced amounts of Fe and Si having received solid melting in supersaturated condition during cast rolling, thus leading to improved foil rolling with eventual formation of an aluminum base foil that offers excellent strength and sufficient foil rolling.
More specifically, the production process for an aluminum base foil cited above is comprised of continuously cast-rolling a hot melt of an aluminum alloy directly into a strip-like cast sheet of smaller than 25 mm in thickness, the aluminum alloy being composed of 0.2 to 0.8% by weight of Fe and 0.05 to 0.3% by weight of Si and the balance of Al and unavoidable impurities, subjecting the cast sheet to cold rolling in an extent of larger than 30% and subsequent heat treatment at a temperature of higher than 400xc2x0 C., cold-rolling the heat-treated sheet at from 250 to 450xc2x0 C. intermediately annealing the cold-rolled sheet, and finally cold-rolling the annealed sheet.
Here, Fe and Si present in the aluminum alloy have a role to render the resultant recrystal grains fine and to make the resultant foil strong. The cold rolling in an extent of larger than 30% and subsequent heat treatment at a temperature of higher than 400xc2x0 C. contemplate scissioning the resulting crystals and breaking the solidified structure into a homogeneous structure such that the finished foil is prevented against rib-like patterns on one of its surfaces to be confronted (a mat surface), and impurities such as Fe, Si and the like are decreased which have been solid-molten while in casting. Consequently, improved foil rolling can be attained. Furthermore, the intermediate annealing treatment conducted at from 250 to 450xc2x0 C. after the second cold rolling is intended to make the recrystal grains fine and, at the same time, to gain improved foil rolling with pinholes prevented from becoming undesirably increased.
However, ribbed patterns exerted on both of the mat and rolled surfaces of the foil are primarily because of the presence of a multilayered phase and the ununiformn or irregular distribution of intermetallic compounds in the course of casting, but not because of the remaining cast structure as will be described later. The two problems need to be solved at one time in alleviating ribbed patterns on the finished foil. In addition, in order to eliminate the irregular distribution of intermetallic compounds, a range of temperatures for heat treatment should be controlled with great precision since metallic compounds of Fe and Si are allowed to deposit at varying temperatures in a 250 to 450xc2x0 C. range.
Strict requirements have lately been made for foils of an Alxe2x80x94Fe alloy type in respect of their surface qualities, and a quality level has been needed which should be favorably compared to that of an aluminum foil attained by the semi-continuous casting method. Namely, it has been demanded that an Alxe2x80x94Fe type alloy foil obtained by the above continuous casting method be improved since the foil is adversely affected by rib-like patterns produced on the rolled and mat surfaces. All of the ribbed patterns appear in a direction of rolling of the foil. The ribbed pattern on the rolled surface of the foil is of a macroscopic nature with a rib width of 2 to 10 nm (hereinafter called a macroscopic rib pattern, and reference made to a photograph of FIG. 1 of the accompanying drawings), whereas the ribbed pattern on the mat surface is of a miniscopic nature with a rib width of 10 to 100 xcexcm (hereinafter called a microscopic rib pattern, and reference made to a photograph of FIG. 2). The two different rib patterns would presumably be due to their respective different mechanisms. No process for the production of an Alxe2x80x94Fe type alloy foil has been proposed to date in which a conventional Alxe2x80x94Fe type alloy foil could be simultaneously prevented against the macroscopic and microscopic rib patterns.
With the aforementined problems of the prior art in view, the present invention seeks to provide a process for the production of a base foil for use as an Alxe2x80x94Fe type alloy foil resulting from a continuous casting method, which base foil is substantially free of macroscopic and microscopic rib patterns and excellent in foil rolling.
As a result of their continued research to avoid the macroscopic and microscopic rib patterns on both surfaces of the foil as experienced in the prior art, the present inventors have found that a continuously cast-rolled plate derived from a hot melt of an Alxe2x80x94Fexe2x80x94Si type alloy, such alloy being obtained by incorporating a sufficient amount of Si in an Alxe2x80x94Fe hot melt, can be formed into a plate of reduced thickness with a substantially a-single phase of AlFeSi (an a-phase of an Alxe2x80x94Fexe2x80x94Si terelement on an Al side), and when an Alxe2x80x94Fe type compound (for example, Al3Fe) and an Alxe2x80x94Fexe2x80x94Si type compound (AlxFeYSi, X, Y are number) are caused to deposit during subsequent cold rolling, a foil can be attained with macroscopic and microscopic rib patterns avoided generally from external view.
According to one aspect of the present invention, there is provided a process for the production of a base foil of an aluminum alloy, which comprises a first heating step in which a cold-rolled plate derived from a continuously cast-rolled plate is heat-treated to promote deposition of an Alxe2x80x94Fe type compound, the cast-rolled plate being comprised of an Alxe2x80x94Fexe2x80x94Si type alloy and having a substantially a-single phase of AlFeSi, and a second heating step in which the resultant plate is heat-treated to promote deposition of an Alxe2x80x94Fexe2x80x94Si type compound after the first heating step.
According to another aspect of the invention, there is provided a process for the production of a base foil of an aluminum alloy, which comprises a first heating step in which a cold-rolled plate derived from a continuously cast-rolled plate is maintained at a temperature between higher than 350xc2x0 C. and lower than 450xc2x0 C. for longer than 0.5 hour, the cast-rolled plate being comprised of an Alxe2x80x94Fexe2x80x94Si type aluminum alloy, the aluminum alloy containing Fe in a content between more than 0.3% by weight and less than 1.2% by weight and Si in a content between more than 0.20% by weight and less than 1% by weight and having a Si/Fe ratio between above 0.4 and below 1.2, and a second heating step in which the resultant plate is maintained at a temperature between higher than 200xc2x0 C. and lower than 330xc2x0 C. for longer than 0.5 hour.